Abstract/Description

When urban areas expand without concomitant increases in wastewater treatment capacity, vast
quantities of wastewater are released to surface waters with little or no treatment. Downstream of many
urban areas are large areas of irrigated croplands reliant on these same surface water sources. Case studies
document the widespread use of untreated wastewater in irrigated agriculture, but due to the practical and
political challenges of conducting a true census of this practice, its global extent is not well known except
where reuse has been planned. This study used GIS-based modeling methods to develop the first spatially
explicit estimate of the global extent of irrigated croplands influenced by urban wastewater flows, including
indirect wastewater use. These croplands were further classified by their likelihood of using poor quality
water based on the spatial proximity of croplands to urban areas, urban wastewater return flow ratios, and
proportion of wastewater treated. This study found that 65 percent (35.9 Mha) of downstream irrigated
croplands were located in catchments with high levels of dependence on urban wastewater flows. These
same catchments were home to 1.37 billion urban residents. Of these croplands, 29.3 Mha were located in
countries with low levels of wastewater treatment and home to 885 million urban residents. These figures
provide insight into the key role that water reuse plays in meeting the water and food needs of people
around the world, and the need to invest in wastewater treatment to protect public health.